In continuation of previous work, we plan to study the various aspects of configuration of the action potential which are important to release of transmitter by the interneuron L10 in the abdominal ganglion of Aplysia. We have observed that the most important modulator of transmitter release in this neuron is the amplitude of the spike after hyperpolarization, the greater the after hyperpolarization the larger the PSPs produced in the follower neurons. This is particularly true when one observes the hyperpolarizing phase of an electrotonic potential in neuron L 20 which is electrotonically coupled to L 10. This neuron then can be used to monitor the action potential of L10 in its branches nearest to the terminal. Voltage clamp techniques will be used to elucidate the role of Ca ions and K ions in the amplitude of the after hyperpolarization and transmitter release. We also plan to study the mechanism involved in rate of desensitization in a follower neuron of L 10, which responds with four types of potentials when L 10 is activated. This neuron L 7, has first a fast desensitizing EPSP which is followed by a fast IPSP, followed by a slow EPSP which at times is yet followed by a slow IPSP. The ionic mechanism of these latter two potentials are the subjects of our study.